For switching control of a magnetic contactor (electromagnetic contactor), an exciting current to be supplied to an operating coil is adjusted to thereby control a main contact, which is in an open circuit condition as a result of charging by a spring force, into a state that can be attracted, and then, the main contact is held and shifted to a closed circuit condition. Drive systems of operating coils include a direct-current voltage-driven type system and an alternating-current voltage-driven type system. Patent Document 1 and 2 have been known as drive circuits for switching a direct-current voltage-driven magnetic contactor to an attracting condition and a holding condition.
Namely, Patent Document 1 discloses an operating circuit of a switch, in which a switch, a switch operating coil, and a field-effect transistor are connected in series, and a gate circuit generates a continuous closing on-gate signal for a fixed time after the switch has been closed, and generates an on/off gate signal for holding a fixed frequency and a predetermined duty ratio after the fixed time has passed, so that the field-effect transistor is turned on and off by an output signal from the gate circuit.
In addition, Patent Document 2 discloses a direct-current electromagnetic device provided with an operating coil, a switching element, and a current detecting resistor connected to the operating coil in series, a one-shot pulse generating circuit that generates a one-shot pulse having a pulse width corresponding to an energization time of a closing coil current of the operating coil, and a comparator for outputting, in an output period of this one-shot pulse, a control signal that controls a coil current of the operating coil to a set current proportional to a power supply voltage to the switching element.
In short, in the conventional technology, a drive circuit of a direct-current voltage-driven magnetic contactor includes a semiconductor switching element and a current detecting resistor connected in series to an operating coil of the magnetic contactor, and an attracting condition and a holding condition have been switched over by controlling the width of an ON-time of the semiconductor switching element.
On the other hand, consider a power converter that temporarily forward-converts an alternating-current power using a rectifier, accumulates the same in a smoothing capacitor, gives a terminal voltage of the smoothing capacitor to a switching circuit including semiconductor switching elements, and inverse-converts the same to an alternating-current power having a frequency required for driving an electric motor. In such a power converter, in order to suppress a great current from flowing into the smoothing capacitor at power-on, an inrush current suppression circuit is provided between the rectifier and the smoothing capacitor. The inrush current suppression circuit is structured so that, immediately after power-on, a current suppression resistor is interposed between the rectifier and the smoothing capacitor to suppress an inrush current, and thereafter, both ends of the current suppression resistor are brought in a short-circuited state by a short circuit. A direct-current voltage-driven magnetic contactor has been used for this short circuit. In the case of the direct-current voltage-driven magnetic contactor, the drive circuit as described above is provided in an accompanying manner.
Patent Document 1: Japanese Published Unexamined Patent Application No. S61-93529
Patent Document 2: Japanese Published Unexamined Patent Application No. H05-291031